[llvm-cov] Clean up an awkward capture-by-reference (NFC)

Writing `for (StringRef &SourceFile : ...)` is strange to begin with.
Subsequently capturing "SourceFile" by reference is even stranger. Just
copy the StringRef, since that's cheap to do.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@275515 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed